JP6570919B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery, and more particularly to a lithium ion secondary battery having good discharge capacity and cycle durability.

  In recent years, in order to cope with air pollution and global warming, reduction of the amount of carbon dioxide has been strongly desired. In the automobile industry, there is a great expectation for reducing carbon dioxide emissions by introducing electric vehicles (EV) and hybrid electric vehicles (HEV). Electric devices such as secondary batteries for motor drive that hold the key to their practical application. Is being actively developed.

  As a secondary battery for driving a motor, it is required to have extremely high output characteristics and high energy as compared with a consumer lithium ion secondary battery used in a mobile phone, a notebook personal computer or the like. Therefore, lithium ion secondary batteries having the highest theoretical energy among all the batteries are attracting attention, and are currently being developed rapidly.

  In general, a lithium ion secondary battery has a positive electrode in which a positive electrode active material or the like is applied on both sides of a positive electrode current collector and a negative electrode in which a negative electrode active material or the like is applied on both sides of the negative electrode current collector is connected via an electrolyte layer. And has a configuration of being housed in a battery case.

Conventionally, carbon / graphite-based materials that are advantageous in terms of charge / discharge cycle life and cost have been used for negative electrodes of lithium ion secondary batteries.
However, since carbon / graphite-based negative electrode materials are charged / discharged by occlusion / release of lithium ions into / from graphite crystals, the charge / discharge capacity of the theoretical capacity 372 mAh / g or more obtained from LiC ₆ which is the maximum lithium-introduced compound. There is a disadvantage that cannot be obtained. For this reason, it is difficult to obtain a capacity and energy density satisfying a practical level of application for vehicles by using a carbon / graphite negative electrode material.

  On the other hand, a battery using a negative electrode active material made of a material that forms an alloy with Li, such as an alloy containing silicon (Si) or a material containing silicon oxide (SiO), is used as a negative electrode. Since the energy density is improved as compared with the battery used for the active material, it is expected as a negative electrode material for vehicle use.

Japanese Patent Application Laid-Open No. 2014-241263 of Patent Document 1 discloses a lithium ion secondary battery using an alloy containing Si as a negative electrode active material.
The Si material absorbs and releases _4.4 mol of lithium ions per mol as shown in the following reaction formula (A) in charge and discharge, and the theoretical capacity is 2100 mAh / g in Li ₂₂ Si ₅ (= Li _4.4 Si). is there. Furthermore, when calculated per Si weight, it has an initial capacity of 3200 mAh / g.

JP 2014-241263 A

  However, a battery using a negative electrode active material containing Si such as an alloy containing silicon (Si) or silicon oxide (SiO) has a discharge capacity and charge compared to a battery using a carbon / graphite-based material as the negative electrode active material. There is a problem that the difference from the capacity, that is, the irreversible capacity is large. In general, the first cycle in which the irreversible capacity is discharged and charged is the largest, and a battery using a negative electrode active material having a large irreversible capacity has a reduced initial capacity and a reduced energy density.

  This invention is made | formed in view of the subject which such a prior art has, The place made into the objective is to provide the lithium ion secondary battery excellent in discharge capacity and cycle durability.

  A lithium ion secondary battery generally has a configuration having a plurality of single battery layers in which a positive electrode, a separator, and a negative electrode are stacked. Therefore, the negative electrode of the single battery layer located in the outermost layer has a positive electrode non-facing surface that does not have an opposing positive electrode.

  Then, two types of a negative electrode having a negative electrode active material layer on both sides and a negative electrode having a negative electrode active material layer only on one side are prepared, and the two types of negative electrodes are selected according to the stacking position of the power generation element. Therefore, a negative electrode active material layer is also formed on the non-positive electrode non-facing surface.

  The present inventors investigated the relationship between the presence or absence of the negative electrode active material layer on the non-positive electrode surface and the irreversible capacity and cycle durability reduction, and the negative electrode active material layer on the non-positive electrode surface is irreversible capacity and cycle durability. I found out that it was one of the causes of the decline.

Specifically, a lithium ion secondary battery in which a positive electrode provided with a positive electrode active material layer on both sides of a positive electrode current collector is provided between two negative electrode electrodes, and a negative electrode active material layer is also provided on a non-positive electrode facing surface. The irreversible capacity and the cycle durability were compared between the formed lithium ion secondary battery and the lithium ion secondary battery from which the negative electrode active material layer on the non-positive electrode facing surface was removed. The evaluation results are shown in Table 1 below.
The Si alloy / graphite of the negative electrode active material layer was 20/80 wt%.

  The discharge capacity and cycle durability can be improved by removing the negative electrode active material layer on the non-positive electrode facing surface from the above evaluation results. However, since the negative electrode active material layer containing silicon (Si) is firmly bonded to the negative electrode active material layer and the current collector, it is difficult to remove the negative electrode active material layer.

  That is, the negative electrode active material layer containing silicon (Si) has a large expansion / contraction due to charge / discharge. For example, the volume expansion when a negative electrode active material layer using a graphite material occludes lithium ions is 1.2 times, whereas Si and Li are alloyed in a negative electrode active material layer using a Si material. At the same time, the transition from the amorphous state to the crystalline state causes a large volume change of about 4 times.

  Therefore, the negative electrode active material layer using the Si material has a short tact time during production because the negative electrode active material layer and the current collector are firmly bonded to prevent the negative electrode active material layer from falling off the current collector. Therefore, it is extremely difficult to completely remove the negative electrode active material layer.

  In addition, in order to ensure a high energy density, an in-vehicle battery often uses a battery with an electrode laminate type exterior laminate structure, and generates power against an external impact or the like by the negative electrode active material layer on the non-positive electrode facing surface. Element protection cannot be ignored.

  Therefore, the present inventors have further studied, and even when a negative electrode active material layer is present on the non-positive electrode facing surface, the negative electrode active material layer is taken out of the charge / discharge reaction system to achieve the above object. The inventors have found that this can be achieved and have completed the present invention.

That is, the lithium ion secondary battery of the present invention includes a negative electrode having a negative electrode active material layer on both sides of a current collector, a positive electrode facing the negative electrode active material layer, and a non-aqueous electrolyte, and the negative electrode The active material contains 0.5 to 15% by mass of silicon.
The negative electrode has a positive electrode non-opposing surface that does not have an opposing positive electrode, the negative electrode active material layer on the non-positive electrode non-opposing surface has an electrolyte repellent layer, and the weight ratio of ethylene carbonate to diethyl carbonate is 1 : The contact angle with respect to the non-aqueous electrolyte containing the mixed solvent of 2 is 25 degrees or more, It is characterized by the above-mentioned.

  According to the present invention, since the electrolyte repellent layer is provided on the negative electrode active material layer formed on the positive electrode non-facing surface of the negative electrode, a reduction in initial capacity is prevented, and the lithium ion secondary battery having excellent cycle durability Can be provided.

It is a schematic sectional drawing which shows an example of the lithium ion secondary battery of this invention.

The lithium ion secondary battery of the present invention will be described in detail.
Hereinafter, a non-bipolar stacked lithium ion secondary battery will be described as an example, but the present invention can also be applied to a bipolar stacked lithium ion secondary battery and a wound lithium ion secondary battery.
FIG. 1 is a schematic cross-sectional view schematically showing the basic configuration of a stacked lithium ion secondary battery.

As shown in FIG. 1, the stacked lithium ion secondary battery 1 has a structure in which a power generation element 10 in which a charge / discharge reaction actually proceeds is sealed inside a battery outer package 20.
The power generation element 10 has a configuration in which a plurality of unit cells in which a positive electrode 12, a separator 11, and a negative electrode 13 are sequentially stacked are stacked.
The separator 11 prevents contact between the positive electrode 12 and the negative electrode 13 and holds a non-aqueous electrolyte solution 14.

The positive electrode 12 has a structure in which a positive electrode active material layer 122 is disposed on both surfaces of a positive electrode current collector 121, and the negative electrode 13 has a negative electrode on both surfaces of a negative electrode current collector 131, similar to the positive electrode 12. The active material layer 132 is disposed.
Specifically, the negative electrode 13, the separator 11, and the positive electrode 12 are laminated in this order so that the positive electrode active material layer 122 and the negative electrode active material layer 132 adjacent to the positive electrode active material layer 122 face each other with the separator 11 interposed therebetween. Thus, the positive electrode 12, the separator 11, and the negative electrode 13 constitute one single cell layer.
Therefore, the lithium ion secondary battery 1 shown in FIG. 3 has a configuration in which a plurality of single battery layers are stacked and electrically connected in parallel.

  And the negative electrode 13 of the single battery layer located in the outermost layer also has a negative electrode active material layer 132 on both surfaces of the negative electrode current collector 131 and has a negative electrode active material layer that does not have an opposing positive electrode.

  The positive electrode current collector 121 and the negative electrode current collector 131 have a positive electrode tab 123 connected to the positive electrode and a negative electrode tab 133 connected to the negative electrode, respectively, and are sandwiched between the end portions of the battery outer package 20. Thus, it has a structure led out to the outside of the battery outer package 20.

  In the lithium ion secondary battery of the present invention, the negative electrode active material layer 132 on the non-positive electrode facing surface includes an electrolyte repellent layer 134. The electrolyte repellent layer 134 prevents contact between the non-aqueous electrolyte 14 and the positive electrode non-opposing negative electrode active material layer 132a, and prevents a battery reaction of the negative electrode active material layer on the non-positive electrode non-facing surface. The electrolyte solution repellent layer 134 may be provided on the surface of the negative electrode active material layer, or may be formed on the surface of the mixture particles constituting the negative electrode active material layer.

  The electrolyte repellent layer 134 can be formed by fluorine plasma treatment, resin coating, and surface fine unevenness processing. Especially, since the said fluorine plasma process can prevent an increase in a weight and volume, it can suppress the reduction | decrease in an energy density. Further, the reaction gas enters the voids of the negative electrode active material layer, reduces the affinity of the surface of the mixture particles constituting the negative electrode active material layer to the non-aqueous electrolyte, and makes contact between the negative electrode active material and the non-aqueous electrolyte. It can be surely prevented.

  In the fluorine plasma treatment, plasma irradiation is performed while introducing a reactive gas containing fluorine or a fluorine compound. By performing the fluorine plasma treatment, fluorine atoms or fluorine compound molecules are introduced onto the surface of the mixture particles to form an electric field solution repellent layer. And since an electric field solution repelling layer has a fluorine-containing group, it comes to show non-affinity with respect to a non-aqueous electrolyte, and a contact with a non-aqueous electrolyte is prevented.

Examples of the gas containing fluorine or a fluorine compound include halogen gases such as CF ₄ , CHF ₃ , C ₂ F ₆ , SF ₆ , and NF ₃ , among which CF ₄ is preferable.

The plasma treatment can be controlled by the gas pressure and the irradiation time. Even if the plasma treatment is a low-pressure plasma treatment in which plasma is radiated in a reduced-pressure atmosphere, or an atmospheric pressure plasma treatment in which plasma is radiated in an atmospheric pressure atmosphere, Although the reduced pressure plasma treatment performed in a reduced pressure atmosphere can increase the fluorine gas concentration, a higher effect can be obtained.
The gas pressure is preferably 0.05 Pa to 200 Pa, and more preferably 10 to 150 Pa. The plasma irradiation time is preferably 1 to 20 seconds.
The fluorine atom introduced by plasma irradiation can be quantified by X-ray photoelectron spectroscopy.

The contact angle of the negative electrode active material layer subjected to the fluorine plasma treatment with respect to the non-aqueous electrolyte is preferably 25 ° or more, and more preferably 40 ° or more.
When the contact angle is 25 ° or more, contact with the non-aqueous electrolyte is prevented.

  The resin coating covers the surface of the negative electrode active material layer and prevents contact between the negative electrode active material layer and the non-aqueous electrolyte solution without allowing the non-aqueous electrolyte solution to penetrate. And a resin tape affixed to the surface of the negative electrode active material layer.

  As the resin used for the coating, those that do not swell or dissolve in the non-aqueous electrolyte can be used. For example, polyethylene terephthalate, polyphenylene sulfide, polypropylene, polystyrene, polycarbonate, polymethyl methacrylate, silicon resin, polytetrafluoro Examples include ethylene, polyetheretherketone, polyacetal, and polyimide.

  The thickness of the resin layer to be applied is not particularly limited as long as it can prevent contact between the negative electrode active material layer and the non-aqueous electrolyte, but is preferably 1 to 5 μm from the viewpoint of energy density.

  Examples of the coating method include spray coating, knife coating, bead coating, and ring coating.

  Examples of the base material of the resin tape include polyethylene terephthalate, polyphenylene sulfide, polypropylene, polystyrene, polycarbonate, polymethyl methacrylate, silicon resin, polytetrafluoroethylene, polyetheretherketone, polyacetal, and polyimide.

  In addition, the adhesive for the resin tape is not particularly limited as long as it does not dissolve or decompose in the electrolytic solution, and does not peel from the negative electrode active material layer even if it comes into contact with the electrolytic solution. it can. Examples thereof include an acrylic adhesive obtained by crosslinking a copolymer of an acrylic acid alkyl ester monomer such as butyl acrylate and a functional group-containing monomer such as hydroxyethyl acrylate.

  The surface fine concavo-convex processing is to form fine concavo-convex on the surface of the negative electrode active material layer, increase the surface surface force and repel non-aqueous electrolyte, and can be processed by an excimer laser or the like.

Hereinafter, each member will be described.
<Negative electrode>
The negative electrode has a negative electrode active material layer formed on the current collector surface, and the negative electrode active material layer is bound to the current collector by a binder.
The negative electrode active material layer may further include other additives such as a conductive additive, a lithium salt, and an ion conductive polymer, as necessary.

(1) Negative electrode active material As the negative electrode active material, an active material containing silicon (Si) and capable of inserting and extracting lithium can be used. Since an active material containing Si alloy and / or SiO has an energy density higher than that of a conventional carbon / graphite negative electrode material, the capacity can be increased, and it is suitable as a negative electrode material for vehicle applications.

(A) Si-containing alloy The Si-containing alloy is not particularly limited as long as the energy density is improved as compared with the carbon / graphite negative electrode material, and Si-M (M is at least 1). Seed metal elements) alloys and the like can be used.

Examples of the Si-M alloy include Si-Ti alloys, Si-Cu alloys, Si-Sn alloys, Si-Al alloys, Si-V alloys, Si-C alloys, Si-Ge alloys. Si-Zn alloy, Si-Nb alloy, Si-Ag alloy, Si-Au alloy, Si-Bi alloy, Si-Cd alloy, Si-Co alloy, Si-Cr, Si-Fe Alloy, Si-In alloy, Si-Mg alloy, Si-Mn alloy, Si-Mo alloy, Si-Ni alloy, Si-Pb alloy, Si-Pt alloy, Si-Sb alloy Binary Si alloys such as Si-W alloys.
Furthermore, a Si—Sn—M alloy (M is at least one of Al, V, and C), a Si—Ti—M alloy (M is at least one of Ge, Sn, and Zn), Si—Zn—. Use ternary Si alloys such as M-based alloys (M is at least one of V, Sn, Al, and C), Si-Al-M alloys (M is at least one of C and Nb), and the like. However, it is not limited to these.
For example, Si—Sn—Ti alloys are amorphous, and are preferable in terms of excellent battery performance, with little structural change associated with Li insertion and desorption. Has excellent properties.

(B) SiO (synthesis method)
The method for producing the SiO is not particularly limited, and a conventionally known method can be appropriately used.

  For example, as a method for producing silicon oxide (SiO) powder, a mixed raw material made of silicon dioxide-based oxide powder is heat-treated in a non-oxidizing atmosphere under reduced pressure to generate SiO vapor, which is condensed in the gas phase. And a method of continuously producing a fine amorphous SiO powder of 0.1 μm or less (Japanese Patent Laid-Open No. 63-103815), or by heating and evaporating raw material silicon to roughen the surface texture of the substrate surface The method of vapor deposition (JP-A-9-110412) and the like are mentioned, but are not limited thereto.

In addition to these, Si powder and SiO ₂ powder are blended at a predetermined ratio as raw materials, and mixed, granulated and dried mixed granulated raw materials are heated in an inert gas atmosphere (830 ° C. or higher) or in vacuum Is heated (1100 ° C. or higher and 1600 ° C. or lower) to generate (sublimate) SiO.
Gaseous SiO generated by sublimation is vapor-deposited on the deposition substrate (substrate temperature is 450 ° C. or more and 800 ° C. or less) to deposit SiO precipitates.
Thereafter, the SiO _x powder is obtained by removing the SiO deposit from the deposition substrate and pulverizing it using a ball mill or the like.
Here, _x of SiO _x is in the range of 1.0 ≦ x ≦ 1.1. Since SiO itself is unstable, it immediately disproportionates to Si and SiO ₂ , so that SiO is in a state where microcrystalline Si and SiO ₂ are mixed. When the amount of Si and SiO ₂ is the same, x = 1, but since the amount of SiO ₂ tends to increase slightly, 1.0 ≦ x ≦ 1.1 as described above.
In the present invention, materials having the same amount of Si and O (or SiO ₂ ) are collectively described as SiO.

The x value of the SiO _x powder can be determined by fluorescent X-ray analysis. For example, it can be obtained by using a fundamental parameter method in fluorescent X-ray analysis using O-Ka rays.
For the X-ray fluorescence analysis, for example, RIX3000 manufactured by Rigaku Corporation can be used. As conditions for the fluorescent X-ray analysis, for example, rhodium (Rh) may be used as a target, the tube voltage may be 50 kV, and the tube current may be 50 mA. Since the x value obtained here is calculated from the intensity of the O-Ka line detected in the measurement region on the substrate, it becomes an average value of the measurement region.

(C) Content of “Si-containing alloy and / or SiO” and BET specific surface area In the present invention, regarding the negative electrode active material, “Si-containing alloy and / or SiO” (also referred to as Si active material) in the negative electrode active material layer. Content) is preferably 0.5% by mass or more. If the amount of Si active material (an alloy containing Si or SiO) in the negative electrode active material layer satisfies the above requirements, the desired effect (cycle) can be achieved without reducing the bond between the Si active material and the binder. This is because the durability, particularly the effect of improving the discharge capacity retention ratio, can be sufficiently exhibited.

  As content of Si active material which can balance high capacity | capacitance and cycling durability, it is preferable that it is 0.5-15 mass%, and it is more preferable that it is the range of 5-10 mass%.

The BET specific surface area of the alloy containing Si and / or SiO (Si active material) is preferably 1 m ² / g or more. If the BET specific surface area has a size that satisfies the above requirements, the number of hydroxyl groups on the surface of the Si active material (alloy containing Si or SiO) particles will not decrease, and the desired effect (cycle durability) This is because, in particular, the effect of improving the discharge capacity maintenance rate can be sufficiently exhibited.
However, if the BET specific surface area is too small, Li insertion / desorption becomes difficult, leading to deterioration of rate characteristics. The area of contact if it is too large and the electrolyte solution is easily becomes decomposed large, is preferably 1-20 m ² / g, and more preferably in the range of 2 to 15 m ² / g.

  The BET specific surface area can be measured by a krypton adsorption BET multipoint method using a pore distribution measuring device ASAP-2020 manufactured by Micromeritics.

  The Si-containing alloy or Si active material such as SiO may be a commercially available material or a produced material, and is not particularly limited.

(D) Negative electrode active material other than “alloy containing Si and / or SiO” The negative electrode active material of the present invention may be an alloy containing Si and / or a negative electrode active material containing SiO. In addition to SiO and SiO, existing active material materials can be used within a range that does not impair the effects of the present invention.

For example, carbon / graphite negative electrode materials such as graphite (graphite), soft carbon, hard carbon, lithium titanate, lithium-transition metal composite oxide (for example, Li ₄ Ti ₅ O ₁₂ ), metal material, lithium alloy negative electrode Materials and the like. In some cases, two or more negative electrode active materials may be used in combination. From the viewpoints of capacity and output characteristics, it is preferable to use a carbon / graphite negative electrode material or a lithium-transition metal composite oxide in combination as the negative electrode active material.

  In particular, graphite is preferable from the viewpoint of battery performance, and since it has a layered structure, the structure accompanying Li insertion / extraction is stabilized, and it is excellent in terms of cycle durability (improvement of discharge capacity retention rate). ing. Of course, negative electrode active materials other than those described above may be used.

(E) Content of negative electrode active material other than “Si-containing alloy and / or SiO” Content of the active material (carbon / graphite negative electrode material) other than Si active material in the negative electrode active material layer described above is 50 -99 mass% is preferable, More preferably, it is 70-97 mass%, Most preferably, it is 80-95 mass%.
This is because, within such a range, the effect of improving the cycle durability, particularly the discharge capacity retention rate, can be sufficiently exhibited without impairing the operational effects of the present invention. Furthermore, if content of active materials other than Si active material is 50 mass% or more, desired charging / discharging capacity | capacitance can be provided. If it is 99% by mass or less, the charge / discharge capacity per unit volume can be increased, and this can greatly contribute to the reduction in size and weight of electrical devices such as negative electrodes and eventually lithium ion secondary batteries.

(F) Average particle size of negative electrode active material The average particle size of the Si active material (Si-containing alloy or SiO) contained in the negative electrode active material layer is not particularly limited. It is preferable to be in the range of 5 to 10 μm, more preferably 0.5 to 5 μm.
If the average particle size of the Si active material is 0.5 μm or more, the particle size is not too small, and the reaction with the electrolytic solution does not become excessive, and the amount of decomposition of the electrolytic solution is remarkably suppressed ( It is excellent in that it can be eliminated.
Moreover, the one where the average particle diameter of Si active material is larger is better. However, if it is too large, it is difficult to diffuse Li into the interior, and the performance is deteriorated. Therefore, if the average particle size of the Si active material is 10 μm or less, the particle size is not too large, so that it is possible to prevent (suppress) the occurrence of particle cracking due to expansion during charge / discharge, Further, it is easy to diffuse Li into the active material, which is excellent in that the battery performance can be improved.

Here, the average particle diameter of the Si active material can be subjected to particle size analysis (measurement) by, for example, SEM (scanning electron microscope) observation or TEM (transmission electron microscope) observation.
In some cases, the Si active material powder (particles) or the cross section thereof includes a powder having an indefinite shape having a different aspect ratio (aspect ratio) instead of a spherical or circular shape (cross sectional shape).
Therefore, the average particle diameter mentioned above is the average value of the absolute maximum length of the cut surface shape of each Si active material powder in the observation image because the shape (or its cross-sectional shape) of the Si active material powder is not uniform. It shall represent. The absolute maximum length means the maximum length among the distances between any two points on the contour line of the Si active material powder (or its cross-sectional shape). In addition, it can obtain | require similarly about the measuring method of another average particle diameter (it may be a secondary particle).

  The average particle diameter of the active material other than the Si active material (Si-containing alloy or SiO) contained in the negative electrode active material layer is not particularly limited, but is preferably 5 to 20 μm, more preferably 5 from the viewpoint of increasing the output. It is in the range of 10 μm. If the average particle size of the active material other than the Si active material (graphite / carbon-based material, etc.) is 5 μm or more, the particle size will not be too small, and the reaction with the electrolytic solution will not be excessive. It is excellent in that the amount of decomposition of the electrolytic solution can be remarkably suppressed (eliminated) and the amount of binder required can be reduced.

  Further, it is better that the average particle diameter of the active material (graphite / carbon-based material, etc.) other than the Si active material is larger. However, if it is too large, it is difficult to diffuse Li into the interior, and the performance is deteriorated. Therefore, if the average particle size of the active material other than the Si active material (graphite, carbon-based material, etc.) is 20 μm or less, it is easy to diffuse Li into the active material, and the battery performance can be improved. Are better.

(2) Binder The binder is added for the purpose of maintaining the electrode structure by binding the active materials or the active material and the current collector.
Although it does not specifically limit as a binder used for a negative electrode active material layer, For example, the following materials are mentioned. Polyethylene, polypropylene, polyethylene terephthalate (PET), polyether nitrile (PEN), polyacrylonitrile, polyimide, polyamide, polyamideimide, cellulose, carboxymethylcellulose (CMC), ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene / butadiene Rubber (SBR), isoprene rubber, butadiene rubber, ethylene / propylene rubber, ethylene / propylene / diene copolymer, styrene / butadiene / styrene block copolymer and hydrogenated product thereof, styrene / isoprene / styrene block copolymer and Thermoplastic polymers such as hydrogenated products, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), tetrafluoroethylene hexafluoropropylene Copolymer (FEP), tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), ethylene / tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene / chlorotrifluoroethylene Fluororesin such as copolymer (ECTFE), polyvinyl fluoride (PVF), vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluorine Rubber (VDF-HFP-TFE fluorine rubber), vinylidene fluoride-pentafluoropropylene fluorine rubber (VDF-PFP fluorine rubber), vinylidene fluoride-pentafluoropropylene-teto Fluoroethylene fluororubber (VDF-PFP-TFE fluororubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluororubber (VDF-PFMVE-TFE fluororubber), vinylidene fluoride-chlorotrifluoroethylene Vinylidene fluoride-based fluororubbers such as epoxy-based fluororubbers (VDF-CTFE-based fluororubbers), and epoxy resins. Among these, polyvinylidene fluoride, polyimide, styrene / butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, polyamide, polyamideimide, and the like can be used. May be.
Especially, it is preferable that it is what contains a styrene-butadiene rubber (SBR) and carboxymethylcellulose (CMC), and a polyimide resin.

  Those containing styrene-butadiene rubber (SBR) and carboxymethylcellulose (CMC) have high elasticity and can maintain the structure even when the electrode expands and contracts, and can improve cycle durability.

  In addition, the polyimide resin is excellent in adhesiveness with the current collector, and the volume change at the time of charging / discharging is relaxed, and the cycle durability can be improved. In particular, the aromatic polyimide resin has excellent solvent resistance and can prevent peeling from the current collector.

  The amount of the binder contained in the negative electrode active material layer is not particularly limited as long as it can bind the active material, but it is preferably 0.5 to 15 mass with respect to the negative electrode active material layer. %, More preferably 1 to 10% by mass, and still more preferably 2 to 5% by mass.

If the amount of the binder contained in the negative electrode active material layer is within the above range, an appropriate amount of the binder can be present at the interface with the current collector. Therefore, even when vibration is applied from the outside and the active material layer is displaced, adhesion, peel resistance, and vibration resistance can be exhibited without causing cohesive failure.
Moreover, when there is too much binder amount, the resistance of a battery will be increased. Therefore, by setting the amount of the binder contained in the negative electrode active material layer within the above range, the active material can be efficiently bound, and the energy density can be obtained by the effect of the present invention described above, that is, by forming a uniform film. And good cycle durability can be further improved.

(3) Conductive auxiliary agent The said conductive auxiliary agent is an additive mix | blended in order to improve the electroconductivity of a positive electrode active material layer or a negative electrode active material layer. Examples of the conductive aid include carbon materials such as carbon black such as acetylene black, graphite, and carbon fiber.

  When the negative electrode active material layer contains a conductive additive, an electronic network inside the active material layer is effectively formed, which can contribute to improvement of the output characteristics of the battery.

The average particle diameter of the conductive additive (chain or fibrous conductive additive such as acetylene black) contained in the negative electrode active material layer is not particularly limited, but is preferably in the range of 20 to 70 μm.
If it is within the above range, it can follow the expansion and contraction due to charging / discharging of the Si active material, so that it can maintain a three-dimensional conductive network at all times and is excellent in that the battery performance can be improved. ing.

(4) Lithium salt Examples of the electrolyte salt (lithium salt) include Li (C ₂ F ₅ SO ₂ ) ₂ N, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO _{3 and the} like.

(5) Ion conductive polymer Examples of the ion conductive polymer include polyethylene oxide (PEO) -based and polypropylene oxide (PPO) -based polymers.

(6) Current collector The current collector has an active material layer disposed on one surface or both surfaces thereof.
As a material constituting the current collector, for example, a conductive resin in which a conductive filler is added to a metal, a conductive polymer material, or a non-conductive polymer material may be employed. A metal is preferably used.

  Examples of the metal include aluminum, copper, platinum, nickel, tantalum, titanium, iron, stainless steel, and other alloys.

Examples of the conductive polymer material include polyaniline, polypyrrole, polythiophene, polyacetylene, polyparaphenylene, polyphenylene vinylene, polyacrylonitrile, and polyoxadiazole.
Since the conductive polymer material has sufficient conductivity without adding a conductive filler, it is advantageous in terms of facilitating the manufacturing process or reducing the weight of the current collector.

Examples of the non-conductive polymer material include polyethylene (PE; high density polyethylene (HDPE), low density polyethylene (LDPE)), polypropylene (PP), polyethylene terephthalate (PET), polyether nitrile (PEN), and polyimide. (PI), polyamideimide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA) , Polyvinyl chloride (PVC), polyvinylidene fluoride (PVdF), and polystyrene (PS).
The non-conductive polymer material has excellent potential resistance or solvent resistance.

  As the non-conductive polymer material or the conductive filler added to the conductive polymer material as required, any conductive material can be used without particular limitation.

  For example, metals, conductive carbon, etc. are mentioned as a material excellent in electroconductivity, electric potential resistance, or lithium ion barrier | blocking property.

  The metal is not particularly limited, but at least one metal selected from the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, Sb, and K, or these metals It is preferable to contain an alloy or metal oxide containing.

  The conductive carbon is not particularly limited, but at least selected from the group consisting of acetylene black, vulcan, black pearl, carbon nanofiber, ketjen black, carbon nanotube, carbon nanohorn, carbon nanoballoon, and fullerene. It is preferable that 1 type is included.

  The amount of the conductive filler added is not particularly limited as long as it is an amount capable of imparting sufficient conductivity to the current collector, and is generally about 5 to 35% by mass.

  The size of the current collector is determined according to the intended use of the battery. For example, if it is used for a large battery that requires a high energy density, a current collector having a large area is used.

  Although there is no restriction | limiting in particular also about the thickness of an electrical power collector, Preferably it is 1-100 micrometers, More preferably, it is 3-80 micrometers, More preferably, it is 5-40 micrometers.

In the case of a battery that is not a bipolar type, a current collector having a plurality of through holes can be used. By using a current collector having a plurality of through holes, the weight of the battery can be significantly reduced and the density can be increased.
Examples of the current collector having the plurality of through holes include a punching metal sheet and an expanded metal sheet.

<Positive electrode>
The positive electrode is obtained by forming a positive electrode active material layer on the current collector surface, and the positive electrode active material layer is bound to the current collector by a binder. The positive electrode active material layer may further contain other additives such as a conductive additive, a lithium salt, and an ion conductive polymer as necessary.

(1) Positive electrode active material Examples of the positive electrode active material include a lithium-transition metal composite oxide, a lithium-transition metal phosphate compound, a lithium-transition metal sulfate compound, and the like. May be.

Among these, from the viewpoint of capacity and output characteristics, lithium-transition metal composite oxides are preferable. Further, Li (Ni—Mn—Co) O ₂ and the transition metal of Li (Ni—Mn—Co) O ₂ are preferable. Those represented by the following general formula (1) partially substituted by other elements (hereinafter, sometimes simply referred to as “NMC composite oxide”) are preferable.

（但し、式中、ａは、Ｌｉの原子比を表し、ｂは、Ｎｉの原子比を表し、ｃは、Ｍｎの原子比を表し、ｄは、Ｃｏの原子比を表し、ｘは、Ｍの原子比を表し、
上記ａ、ｂ、ｃ、ｄ、ｘは、
０．９≦ａ≦１．２
０＜ｂ＜１
０＜ｃ≦０．５
０＜ｄ≦０．５
０≦ｘ≦０．３
ｂ＋ｃ＋ｄ＝１を満たす。
ＭはＴｉ、Ｚｒ、Ｎｂ、Ｗ、Ｐ、Ａｌ、Ｍｇ、Ｖ、Ｃａ、Ｓｒ、Ｃｒから選ばれる元素で少なくとも１種類である）

(Wherein, a represents the atomic ratio of Li, b represents the atomic ratio of Ni, c represents the atomic ratio of Mn, d represents the atomic ratio of Co, and x represents M Represents the atomic ratio of
The above a, b, c, d, x are
0.9 ≦ a ≦ 1.2
0 <b <1
0 <c ≦ 0.5
0 <d ≦ 0.5
0 ≦ x ≦ 0.3
b + c + d = 1 is satisfied.
M is at least one element selected from Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr)

The NMC composite oxide has a layered crystal structure in which lithium atomic layers and transition metal (Mn, Ni, and Co are arranged in order) atomic layers are alternately stacked via oxygen atomic layers. One atom of transition metal contains one Li atom, and the amount of Li that can be taken out is twice that of spinel-based lithium manganese oxide, that is, the Li supply capacity is twice, so that it has a high capacity. it can.
The composition of each element can be measured by, for example, inductively coupled plasma (ICP) emission spectrometry.

  The average particle size of the positive electrode active material contained in the positive electrode active material layer is not particularly limited, but is preferably 1 to 100 μm, more preferably 1 to 20 μm from the viewpoint of increasing the output.

(2) Binder The binder used for the positive electrode active material layer is not particularly limited, and the same binder as the negative electrode can be used.

  The amount of the binder contained in the positive electrode active material layer is not particularly limited as long as it is an amount capable of binding the active material, but is preferably 0.5 to 15 mass with respect to the positive electrode active material layer. %, Preferably 1 to 10% by mass, more preferably 2 to 6% by mass.

  About other additives other than a binder, the thing similar to the column of the said negative electrode active material layer can be used.

<Non-aqueous electrolyte>
The nonaqueous electrolytic solution is obtained by dissolving a lithium salt in an organic solvent, and has a function as a lithium ion carrier.

As the lithium _{salt, Li (CF 3 SO 2)} 2 N, Li (C 2 F 5 SO 2) 2 N, LiPF 6, LiBF 4, LiClO 4, LiAsF 6, LiTaF 6, LiCF 3 SO 3 or the like of the electrode A compound that can be added to the active material layer can be similarly employed.

  Examples of the organic solvent include carbonates such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), fluoroethylene carbonate (FEC), and ethyl methyl carbonate. I can.

The non-aqueous electrolyte may be injected into a matrix polymer (host polymer) made of an ion conductive polymer.
By being injected into the matrix polymer, the fluidity of the non-aqueous electrolyte is lowered, and it becomes easy to block the ion conductivity between the layers.
Moreover, the matrix polymer can express the outstanding mechanical strength by forming a crosslinked structure.

  Examples of the ion conductive polymer include polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof.

<Separator>
The separator has a function as a partition that holds the non-aqueous electrolyte, ensures lithium ion conductivity between the positive electrode and the negative electrode, and prevents a short circuit between the positive electrode and the negative electrode.

  Examples of the separator include a porous sheet separator and a nonwoven fabric separator made of a polymer or fiber that absorbs and holds a non-aqueous electrolyte.

  Examples of the separator for the porous sheet made of the above-described polymer or fiber include polyolefins such as polyethylene (PE) and polypropylene (PP); a laminate in which a plurality of these are laminated (for example, a three-layer structure of PP / PE / PP) Laminates, etc.), hydrocarbons such as polyimide, aramid, polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP), and microporous (microporous membrane) separators made of glass fibers.

  As said nonwoven fabric separator, conventionally well-known things, such as cotton, rayon, acetate, nylon, polyester; Polyolefins, such as PP and PE; Polyimide, Aramid, etc. can be used individually or in mixture.

  The thickness of the separator depends on the use application, but for example, in applications such as secondary batteries for driving motors such as electric vehicles (EV), hybrid electric vehicles (HEV), and fuel cell vehicles (FCV), It is desirable that it is 4-60 micrometers, and a single layer structure or a multilayer structure may be sufficient.

<Battery exterior>
The said battery exterior body is a member which encloses an electric power generation element in the inside, is a bag-like case which can cover an electric power generation element, and a laminate film is used.

As the laminate film, for example, a laminate film having a three-layer structure in which PP, aluminum, and nylon are laminated in this order can be used.
The laminated film can increase the energy density.

  Hereinafter, although it demonstrates still in detail using an Example and a comparative example, this invention is not necessarily limited to the following Examples at all.

[Comparative Example 1]
(Preparation of negative electrode)
A Si—Ti alloy was produced by a mechanical alloy method.
Specifically, 80 wt% Si raw material and 20 wt% Ti raw material are put into a zirconia crushing pot, zirconia crushing balls are added to this, and a planetary ball mill apparatus (manufactured by German company Frichtu: P-6) is used. And mixed at 600 rpm and alloyed for 24 hours.
Thereafter, pulverization was performed at 400 rpm for 1 hour to obtain a Si—Ti alloy powder.
The obtained Si—Ti alloy powder had an average particle size (average particle size of secondary particle size) of 8 μm and a BET specific surface area of 14 m ² / g.

19.2 wt% of the Si-Ti alloy powder, graphite powder (average particle size 20 μm, BET specific surface area 4 m ^{2 /} g) 76.8 wt%, acetylene black (conducting aid: average particle size 50 nm) 1 wt%, and aqueous binder (SBR / CMC = 2/1) 3 wt% was added to water and mixed to prepare a negative electrode slurry.
The negative electrode slurry was applied to both sides of a copper foil (thickness 10 μm) and dried at 130 ° C. for 24 hours to produce a negative electrode.

(Preparation of positive electrode)
Lithium-transition metal sulfate compound (Li _1.85 Ni _0.18 Co _0.10 Mn _0.87 O ₃ : average particle size 20 μm) 90 wt%, acetylene black (conducting aid: average particle size 50 nm) 5 wt%, and Polyvinylidene fluoride (PVDF) 5 wt% was added to N-methylpyrrolidone and mixed to prepare a positive electrode slurry.
The positive electrode slurry was applied to both surfaces of an aluminum foil (thickness: 15 μm) and sufficiently dried to form a positive electrode active material layer having a thickness of 50 μm to produce a positive electrode.

(Preparation of non-aqueous electrolyte)
A lithium salt (LiPF ₆ ) was added to a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (weight ratio = 1: 2) to prepare a 1M LiPF ₆ nonaqueous electric field solution.
Here, “1.0M LiPF ₆ ” means that the lithium salt (LiPF ₆ ) concentration in the mixture (electrolytic solution) of the solvent and the lithium salt is 1M.

(Production of battery)
The positive electrode active material layer of the positive electrode and the negative electrode active material of the negative electrode are opposed to each other with a separator (polypropylene microporous film: thickness 20 μm: porosity 30%), and the negative electrode and the positive electrode are alternately arranged. A power generation element having three layers of negative electrodes and two layers of positive electrodes was fabricated.
In addition, the said electric power generation element has a positive electrode non-opposing negative electrode active material layer which does not have a positive electrode active material layer which opposes both end surfaces.

  A tab is welded to each of the positive electrode and the negative electrode of the power generation element, placed in a cell container (made of an aluminum laminate film), the non-aqueous electrolyte is liquid-sealed in a vacuum, and a lithium ion secondary battery is sealed. Obtained.

[Example 1]
A lithium ion secondary battery was produced in the same manner as in Comparative Example 1 except that the negative electrode active material layer on the non-positive electrode facing surface that did not have the positive electrode active material layer of the power generation element was treated with fluorine plasma under the following conditions.
Equipment: Sakai Semiconductor Co., Ltd .: YHS-G
Treatment time: 3 seconds Reactive gas: Fluorine (F ₂ ) 99.999%
Reaction pressure: 100 Pa

[Example 2]
A lithium ion secondary battery was produced in the same manner as in Example 1 except that the reactive gas for the fluorine plasma treatment was changed to carbon tetrafluoride (CF ₄ ).

[Example 3]
A lithium ion secondary battery was produced in the same manner as in Example 2 except that the treatment time of the fluorine plasma treatment was changed to 5 seconds.

[Example 4]
A lithium ion secondary battery was produced in the same manner as in Example 3 except that the reaction pressure of the fluorine plasma treatment was changed to 10 Pa.

[Example 5]
The same as Comparative Example 1 except that a polyimide film (Kapton tape (registered trademark)) having a thickness of 30 μm was attached to the entire surface of the positive electrode non-opposing negative electrode active material layer of the two negative electrodes located in the outermost layer of the power generation element. Thus, a lithium ion secondary battery was produced.

[Evaluation]
The contact angle of the negative electrode active material layer of the lithium ion secondary battery with respect to the non-aqueous electrolyte and the battery characteristics were evaluated by the following methods. The evaluation results are shown in Table 2.

(Measurement of contact angle)
The contact angles of the positive electrode non-opposing negative electrode active material layer and the negative electrode active material layer with respect to the non-aqueous electrolyte solution were measured.
The contact angle was measured using an automatic minimum contact angle meter (manufactured by Kyowa Interface Science Co., Ltd .: MCA-3), 1 μL of the non-aqueous electrolyte was dropped, and the contact angle after 30 seconds was measured.
The said measurement was performed 5 times and the average value was made into the contact angle.

(Evaluation of battery characteristics)
The lithium ion secondary battery was charged and discharged in a constant temperature bath at 27 ° C., and the battery characteristics were evaluated by a charge / discharge tester (Hokuto Denko Co., Ltd .: HJ0501SM8A).
Specifically, it was charged in constant current / constant voltage mode at 2.5 mA up to 4.2 V, and pretreated by discharging in constant current mode from 4.2 V to 2.5 V at 2.5 mA equivalent. .
Thereafter, the current value is increased to 12.5 mA, and charging (Li insertion process into the negative electrode active material layer) is performed, and then discharge (Li desorption process from the negative electrode active material layer) is performed. The charge / discharge cycle at a value of 12.5 mA was defined as 1 cycle, and this was repeated 100 times, and the discharge capacity retention rate at the 100th cycle was measured.

  The “discharge capacity retention rate (%)” is the discharge capacity at the 100th cycle relative to the discharge capacity at the first cycle, and is calculated by the following equation (2).

  Further, the discharge energy at the first cycle was divided by the volume including the cell exterior to calculate the volume energy density (mAh / cc), and the volume energy density of the lithium ion secondary battery of Comparative Example 1 was set to 100%. The volume energy density of the example lithium ion secondary battery was calculated.

Comparative Examples 1 in which the lithium ion secondary batteries of Examples 1 to 4 did not form the electrolyte repellent layer by treating the negative electrode active material layer on the non-positive electrode facing surface with fluorine plasma to form the electrolyte repellent layer Compared to the above, the discharge capacity and the discharge maintenance ratio in the first cycle were improved.
From the above evaluation results, it can be seen that the effect on the battery performance is increased by using CF ₄ as the reaction gas, lowering the reaction gas pressure, and increasing the treatment time.
In Example 5, the negative electrode active material layer on the non-positive electrode non-facing surface was covered with a tape, so that the discharge maintenance ratio was improved as compared with Comparative Example 1. However, the discharge capacity at the first cycle was reduced due to the increase in the volume of the tape. .
Moreover, Example 5 has a discharge maintenance rate lower than Examples 1-4. This is considered to be because the end face of the positive electrode non-opposing negative electrode active material layer could not be completely covered, contact with the non-aqueous electrolyte occurred, and the adhesive was dissolved in the non-aqueous electrolyte.

DESCRIPTION OF SYMBOLS 1 Stacked type lithium ion secondary battery 10 Power generation element 11 Separator 12 Positive electrode 121 Positive electrode current collector 122 Positive electrode active material layer 123 Positive electrode tab 13 Negative electrode 131 Negative electrode current collector 132 Negative electrode active material layer 133 Negative electrode tab 134 Electrolyte repellent layer 14 Non-aqueous electrolyte 20 Battery exterior body

Claims (3)

A lithium ion secondary battery comprising a negative electrode having a negative electrode active material layer, a positive electrode having a positive electrode active material layer facing the negative electrode active material layer, and a non-aqueous electrolyte,
The negative electrode active material contains 0.5 to 15% by mass of silicon,
The non-aqueous electrolyte contains a mixed solvent having a weight ratio of ethylene carbonate to diethyl carbonate of 1: 2 .
The negative electrode includes a negative electrode active material layer on a positive electrode non-facing surface that does not have a positive electrode active material layer facing the negative electrode,
The lithium ion secondary battery, wherein the negative electrode active material layer on the non-facing surface of the positive electrode includes an electrolyte repellent layer having a contact angle with respect to the nonaqueous electrolyte of 25 ° or more .   The lithium ion secondary battery according to claim 1, wherein the electrolyte repellent layer has a fluorine-containing group. 3. The lithium ion secondary battery according to claim 2, wherein the negative electrode active material layer has been subjected to fluorine plasma treatment with fluorine gas or CF ₄ gas .

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